ation. In the Octopoda and in
_Sepia_, _Sepiola_ and _Rossia_, each egg has a separate envelope
continued into a long stalk by which it is attached with several
others in a cluster. In _Argonauta_ the eggs are carried by the female
in the cavity of the shell. In _Loligo_ the eggs are very numerous,
and are enclosed in cylindrical transparent gelatinous strings united
at one end into a cluster.

The Cephalopoda appear to be the only Invertebrates in which the egg
is mesoblastic and telolecithal like that of Vertebrata. This is the
result of the large quantity of the yolk, and the position the latter
assumes in relation to the blastoderm. In all other Mollusca the
segmentation is complete though in some cases very unequal. In the egg
of _Loligo_, which has been chiefly studied (fig. 35), the
protoplasmic pole is at the narrower end of the egg, and segmentation
is restricted to this end, forming a layer of ectoderm cells. From one
part of the periphery of the ectoderm proliferation of cells takes
place and gives rise to a layer of scattered nuclei over the whole
surface of the yolk. The region of proliferation marks the anal side
of the ectoderm, and the layer of nuclei forms the perivitelline
membrane. This process must be regarded as equivalent to the first
stage of invagination, the yolk being surrounded by hypoblast cells or
their nuclei. Later on the same anal edge of the ectoderm forms
another cellular layer, the endoderm proper, which forms a continuous
sheet below the ectoderm.

The mesoderm also originates at the anal side of the ectoderm and
extends in two bands right and left between ectoderm and endoderm.
After the mesoderm is thus established, a little vesicle lying upon
and open to the yolk is formed from the endoderm, and this vesicle
ultimately gives rise to the stomach, the two lobes of the liver and
the intestine. The buccal mass and oesophagus arise from a stomodaeal
invagination, and the anus is formed later from a short proctodaeal
invagination.

The external changes of form are as follows:--The mantle is the middle
of the embryonic area, and in its centre is the shell-gland, which,
however, behaves in a different way from that seen in other Molluscs.
Its borders grow inwards and approach each other to form the
shell-sac. E. Ray Lankester showed that in _Argonauta_ and other
Octopods the shell-sac disappears before it is close